For the cleaning of chemical-process and municipal or other waste water containing heavy metals as well as solids and dissolved impurities, it is generally impractical, if not impossible, to eliminate all of the impurities which may be considered to be environmental contaminants in a single stage. Generally speaking, the removal of suspended solids from the liquid leaves a liquid phase which can be treated without significant difficulty and also makes possible the recovery of valuable components from the separated solid phase.
The solids can be separated from liquids in which they are entrained by sedimentation (settling of solids) and decantation of the supernatant liquid or by holding back the solids while permitting the liquid to pass on a sieve or other filtering surface. Reference to a filtering apparatus, therefore, will mean an apparatus having a surface upon which solids can be collected or on which solids can be held back while the liquid is permitted to pass. Sedimentation takes a disadvantageously long time and is seldom quantitative especially where the particle size distribution in the liquid phase is wide. However, filtration methods for low solids loading of a liquid, e.g. a solids loading below 1% by weight, are comparatively expensive since a sufficient filtration rate requires large filter surfaces and large filter surfaces imply high capital and process costs. The latter are, of course, a result of the need to maintain a certain pressure differential across the solids-collecting surface during the vibration process, the need to remove the filter cake from time to time and the need to backwash the filter.
Both sedimentation and filtration, therefore, are discontinuous processes which must be interrupted from time to time to allow a cleaning operation to be carried out and during which the liquid to be cleaned must be passed through a further similar apparatus in parallel to the one being cleaned, thereby further increasing the capital cost.